A DC-DC converter is used in an electric power supply system of portable information terminals including a cellular phone as a typical one. There are a variety of requests for the DC-DC converter as to the battery configuring the power source thereof, a control function for the battery, and a voltage conversion function for converting the power supplied from the battery to have a desired voltage, as will be described hereinafter. First request is to increase the retention time length of the battery. More specifically, it is requested to increase the time length, as much as possible, for supplying the power to the portable information terminal per a single cell in the case of the same primary battery, or per a single battery charge in the case of the same secondary battery. For this purpose, it is necessary to output the energy stored in the battery as much as possible and without leaving a remaining energy. For example, although the terminal voltage of the battery falls if the remaining content of the battery is reduced, the voltage conversion function, if any, that allows output of a desired voltage in spite of a lower input voltage will meet this request. However, in such a case, loss associated with the voltage conversion function must be suppressed to a minimum. That is, an efficient voltage conversion function is needed. In addition, use of the secondary battery requires storage of a larger amount of energy per a battery charge, and thus requires a charge function for achieving this storage.
Second request for the battery is to have a smaller size in particular for improvement of the portability thereof. If the size of battery is reduced, the battery capacity is inevitably reduced due to the trade-off. Accordingly, the above function for increasing the retention time length of the battery is desired again even for the purpose of reducing the size of the battery.
Third request is to output a plurality of different voltages and to control the stop of the output thereof. The potable information terminal requires a plurality of power sources used for, in addition to the typical information processing, the interface connecting to a network, analog signal processing such as video and audio signal processings, and in some case a drive unit such as a hard disk drive, which operate typically on different voltages. Moreover, it is preferable that those power sources be capable of being turned OFF for saving the power dissipation when the corresponding functions are not needed.
Fourth request is to have a variable function of the output voltage in some cases. In general, the power dissipation in a digital circuit is proportional to the operating-clock frequency and square of the source voltage. Accordingly, it is generally employed to reduce the operating-clock frequency for saving the power dissipation when a higher information-processing performance is not needed. In addition, since it is generally possible to operate the circuit even on a lower voltage under the lower operating-clock frequency, the source voltage is reduced for achieving a further power saving. Recently, the process for manufacturing semiconductor devices has developed a finer patterning rule, which maintains the existing high-operational speed even on a further reduced voltage.
In an analog circuit, the source voltage of a final-stage transistor in a power amplifier (PA) of a radio interface, for example, is the main factor based on which the maximum output electric power is to be determined. On the contrary, if a higher output electric power is not needed, the source voltage of the final-stage transistor is lowered to achieve a power saving. In the analog circuit, a higher output power is required along with the increase in the amount of transmitted information, contrary to the digital circuit. The increase of the amount of transmitted information requires the analog signal processing to have a lower distortion than ever. These facts require a higher source voltage than ever.
Fifth request is to cope with a wider range of variation in the battery voltage and a lower output voltage of the battery. The battery voltage is not always fixed at constant during the use thereof. Accordingly, the power source circuit must maintain a desired fixed output voltage that does not depend on the battery voltage, whereas the power source circuit is required to operate on a battery voltage lower than ever for the purpose of efficiently using out the battery capacity.
In these days, step-down DC-DC converters shown in FIG. 4 are used as the electric power supply system having a higher power conversion efficiency. FIG. 4(a) is the basic circuit diagram of a switch-type step-down DC-DC converter using a diode. In this figure, the output of battery 208 is connected to a choke coil 102 and diode 602 through a switch 101. Switch 101 performs an ON-OFF operation for the output of battery 208 based on the output of a DC/DC converter controller 601. When switch 101 is turned ON, current flows into choke coil 102 and is passed to a load side, and there arises a back electromotive force accompanying the same, whereby energy is stored in choke coil 102. Diode 602 is in an OFF state.
Subsequently, switch 101 is turned OFF, whereby there occurs a back electromotive force in choke coil 102, which is opposite in the direction to that during the ON state of switch 101. Thus, current flows from the ground via diode 602 and choke coil 102 to the load side for the output thereof. At this stage, diode 602 is in an ON state. Control for ON/OFF time ratio of switch 101 controls the output voltage of the converter. This output voltage is also fed to the DC/DC converter controller 601, and compared therein against a reference voltage (not shown) to obtain an error voltage, which is used for control of the output voltage. Since this technique uses the ON or OFF state of switch 101, there is substantially no power dissipation therein, thereby achieving a higher conversion efficiency.
FIG. 4(b) is such that diode 602 in FIG. 4(a) is replaced by a switch 301, which is controlled by a DC/DC converter controller 603. Switch 301 is controlled to be turned OFF when switch 101 is ON, and controlled to be turned ON when switch 101 is OFF, whereby switch 301 operates similarly to diode 602 in FIG. 4(a). In the circuit of FIG. 4(a), the electric power expressed by a product of the forward-biased voltage of the diode and the current flowing therethrough is consumed in diode 602 when diode 602 is ON, whereas in the circuit of FIG. 4(b), use of a MOSFET etc. as switch 301 further reduces the power dissipation during the ON state. As a result, the conversion efficiency of the DC-DC converter can be improved, while achieving suppression of heat generation. In this technique, if a lower output voltage is desired, the control of lowering the output voltage can be obtained at a higher speed, by turning switch 301 ON to easily evacuate the charge stored on the load capacitor, such as a capacitor 103, to the ground plane.
As described heretofore, if a voltage lower than the battery voltage is to be generated from a battery, the switch-type step-down DC-DC converter can perform an efficient voltage conversion. However, in these days, there are an increasing number of circuits that require a source voltage higher than the battery voltage, as described before, and the step-down DC-DC converter, which cannot deliver an output voltage higher than the battery voltage, cannot deal with such a case.
Thus, there may be considered a technique wherein at least two batteries are connected in series to deliver a higher output voltage, which is efficiently converted in the voltage thereof by using the step-down DC-DC converter. However, in this case, the range of variation in the capacity between the serially connected batteries causes that the overall discharge capacity of the batteries (quantity of the electricity which can be discharged from the batteries) connected in series during the discharge is determined by the capacity of the battery having a lowest discharge capacity, whereby the discharge capacity of all the batteries cannot be effectively used out. In the case of secondary batteries, the overall charge capacity of the batteries (quantity of the electricity which can charge the secondary batteries) connected in series during the charge is determined by the secondary battery having a lowest charge capacity, whereby the charge capacity of the individual secondary batteries cannot be used out. If batteries having a variety of discharge capacities or charge capacities are combined together, an over discharge and an overcharge will likely to occur in the battery having a lowest discharge capacity and the battery having a lowest charge capacity, respectively, thereby causing degradation of the battery. If the batteries are subjected to selection for sorting to reduce the range of variation, it raises the cost. Moreover, since the characteristics vary with aging etc., the range of variation occurs after the selection for sorting.
In consideration of the above, step-up DC-DC converters shown in FIG. 5 are cited as devices that can deliver, from a single battery, an output voltage higher than the battery voltage. FIG. 5(a) shows the basic circuit of the step-up DC-DC converter using a diode.
In FIG. 5(a), the output of battery 208 is connected to switch 701 and diode 704 through choke coil 702. Switch 701 intermittently connects the output-side terminal of choke coil 702 to the ground due to the control by a DC/DC converter controller 705. When switch 701 is turned ON, current flows into choke coil 702, accompanying generation of a back electromotive force, and at the same time, storage of energy in choke coil 702. At this stage, diode 704 is in an OFF state. When switch 701 is subsequently turned OFF, an electromotive force which is opposite to that generated upon the ON state of switch 701 is generated in choke coil 702, whereby a voltage that is a sum of the voltage of battery 208 and the electromotive force of choke coil 702 is output through diode 704. At this stage, diode 704 is in an ON state. The output voltage can be adjusted by controlling the ON/OFF time ratio of switch 701. The output voltage is delivered also to the DC/DC converter controller 705, whereby the output voltage is adjusted by using an error voltage that is obtained by comparing the output voltage against a reference voltage (not shown).
FIG. 5(b) is such that diode 704 shown in FIG. 5(a) is replaced by a switch 706, which is controlled by the DC/DC converter controller 707. Switch 706 is controlled to be turned OFF when switch 701 is ON, and to be turned ON when switch 701 is OFF, whereby switch 706 operates similarly to diode 704 shown in FIG. 5(a). When diode 704 is ON in the circuit of FIG. 5(a), an electric power that is expressed by a product of the forward biased voltage of the diode and the current flowing therethrough is consumed in diode 704, whereas in the circuit shown in FIG. 5(b), use of a MOSFET etc. as switch 706 further reduces the power dissipation during the ON state. As a result, the conversion efficiency of the DC-DC converter can be improved further, while suppressing generation of heat. Since the voltage drop in switch 706 can be lowered compared to the forward biased voltage drop of diode 704, a higher output voltage can be obtained.
However, in the step-up DC-DC converters of FIG. 5, an output voltage that is lower than the battery voltage cannot be delivered. FIG. 6 shows a step-up/down DC-DC converter wherein the switch-type step-down and step-up DC-DC converters are combined together as a technique for solving the above problem, wherein some of the circuit sections are shared therebetween.
In FIG. 6, if the desired output voltage is lower than the battery voltage, switches 101 and 301 and choke coil 102 operate as a switch-type step-down DC-DC converter similarly to the converter of FIG. 4(b). At this stage, switch 701 is OFF and switch 706 is ON. On the other hand, if the desired output voltage is higher than the battery voltage, choke coil 102 and switches 701 and 706 operate as a step-up DC-DC converter similarly to the converter of FIG. 5(b). At this stage, switch 101 is ON and switch 301 is OFF.
As described heretofore, when the step-up/down DC-DC converter shown in FIG. 6 is used, the range of output voltage is not restricted by the battery voltage and thus may be set to a wider range. However, there are problems as recited hereinafter.
First, the value of current output from the battery is higher than the output current. This is because energy is stored in choke coil 102 and thus the current is not output during the ON state of switch 701, for delivering the current during the remaining period. Thus, the burden on the battery is increased. This is conspicuous at the stage of starting the step-up/down converter and the stage of raising the output voltage.
In addition, when the output voltage is in the vicinity of the battery voltage, switching occurs between the step-up mode and the step-down mode, which fact increases the number of operations for evacuating the charge to the ground, thereby reducing the power conversion efficiency.
As described above, if it is desired to obtain a higher voltage than the battery voltage, there are problems in the conventional electric power supply system, which will be recited hereinafter.
The first problem is such that if at least two batteries connected in series are used in order to obtain the higher voltage, it impossible to effectively use out the discharge capacity of all the batteries. This is because the overall discharge capacity of the batteries is determined by the battery having a smallest discharge capacity due to the range of variation in the discharge capacity among the batteries connected in series.
The second problem is such that if at least two secondary batteries connected in series are used in order to obtain the higher voltage, it is impossible to effectively use out the overall charge capacity of all the batteries. This is because the overall charge capacity of the batteries is determined by the battery having a smallest charge capacity due to the range of variation in the charge capacity among the secondary batteries connected in series.
The third problem is such that if at least two secondary batteries connected in series are used in order to obtain the higher voltage, batteries are likely to be deteriorated. This is because batteries having a range of variation in the discharge capacity or charge capacity are combined together, and accordingly an over discharge is likely to occur in the battery having a lowest discharge capacity and an overcharge is likely to occur in the battery having a lowest charge capacity.
The fourth problem is such that it is difficult to select batteries for sorting in order to suppress the range of variation in the discharge capacity or charge capacity for the batteries that are connected in series to obtain the higher voltage. This is because selection of the batteries for sorting raises the cost. In addition, the characteristics vary along with the aging, and the range of variation occurs even after the selection for sorting.
JP-2002-345161A describes a technique wherein a battery block configured by a plurality of batteries (cells) is subjected to measurement of the cell voltage for each cell, and if the cell voltage is higher than the average voltage, a discharge switch for the cell is turned ON, to thereby discharge the cell current through a discharge resistor and equalize the cell voltages. In this technique, however, electric power of the batteries is wasted.
The fifth problem is such that if a step-up DC-DC converter is employed instead of connecting a plurality of secondary batteries in series to thereby obtain the higher voltage, it is impossible to output a voltage lower than the battery voltage, whereas if a step-up/down DC-DC converter is employed, the power conversion efficiency is lowered at a voltage in the vicinity of the battery voltage.